Pakistani activists rally in Karachi Pakistan to condemn killings of health workers, on Thursday, Dec. 20, 2012. Thousands of polio workers escorted by police guards pressed on with a vital immunization program in Pakistan's second largest city Thursday despite the deaths of nine people gunned down by suspected militants while working on the campaign elsewhere in the country. Placard on top reads "save Pakistani children." (AP Photo/Fareed Khan)

Under police guard, thousands of health workers pressed on with a polio immunization program Thursday after nine were killed elsewhere in Pakistan by suspected militants who oppose the vaccination campaign.

Immunizations were halted in some parts of Pakistan and the United Nations suspended its field participation everywhere until better security was arranged for its workers. The violence risks reversing recent progress fighting polio in Pakistan, one of three countries in the world where the disease is endemic.

The Taliban have denied responsibility for the shootings. Militants have accused health workers of acting as spies for the United States, alleging the vaccine is intended to make Muslim children sterile.

Taliban commanders in Pakistan's troubled northwest tribal region also said earlier this year that vaccinations can't go forward until the United States stops drone strikes in the country.

Insurgent opposition to the campaign grew last year after it was revealed that a Pakistani doctor ran a fake vaccination program to help the CIA track down and kill al Qaeda founder Osama bin Laden, who was hiding in the town of Abbottabad in the country's northwest.

There were a few attacks on polio workers in July, but the current level of violence is unprecedented. A polio worker died Thursday after being shot in the head in the northwestern city of Peshawar a day earlier, said health official Janbaz Afridi.

His death raised to nine the number of Pakistanis working on the campaign who have been killed this week. Six of the workers gunned down were women, three of whom were teenagers. Two other workers were critically wounded.